Relation between risk and incidence rate

Risk or Rate?

It is easier to interpret risks than rates. Therefore, it might sometimes be convenient to convert an incidence rate into a risk using the following formula :

Risk = Rate x Time

Suppose that we have a population of 1000 persons in which the incidence rate of cancer X is 6 cases per 1000 person-years (6 / 1000 yr-1). If we follow this population for 30 years, the risk of cancer X in the population over that 30 years is 6 / 1000 yr-1 x 30 years = 0.18 or 18%. Among the 1000 persons present at the start of the follow-up, 180 cases of cancer X will occur. If the follow-up were 15 years, the related risk would be 9 %. The above formula does not consider the decrease of the population at risk over time and cannot be used when the risk is significant. It also assumes that the rate remains constant over time.

Credits

Fem Editor 2007

• Sabrina Bacci

Original Authors

• Alain Moren
• Marta Valenciano
• Arnold Bosman